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Fix BSS segment corruption in file-backed mmap (dynamic library loading) #331

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Merged
Pryancito merged 1 commit into from
Apr 30, 2026
Merged

Fix BSS segment corruption in file-backed mmap (dynamic library loading) #331

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10 changes: 8 additions & 2 deletions eclipse_kernel/src/filesystem.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2501,9 +2501,14 @@ impl Scheme for FileSystemScheme {
if offset >= file_size {
return Err(scheme_error::EINVAL);
}
// `want` is how many bytes actually exist in the file for this mapping.
// `len` may be larger (memsz > filesz) when the segment has a BSS extension.
// We must allocate pages for the full `len` so that the BSS region is backed
// by zeroed physical memory rather than whatever happened to follow the
// file-data allocation in the physical frame pool.
let max_len = file_size - offset;
let want = core::cmp::min(len, max_len);
let aligned_len = (want + 0xFFF) & !0xFFF;
let aligned_len = (len + 0xFFF) & !0xFFF;
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Copilot AI Apr 30, 2026

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aligned_len is computed with (len + 0xFFF) & !0xFFF, which can overflow/wrap for very large len values (release builds wrap). Since len ultimately comes from userspace mmap, this can lead to allocating/zeroing the wrong amount of memory. Use a checked/saturating align-up (e.g., len.checked_add(0xFFF) + mask) and return EINVAL on overflow.

Suggested change
let aligned_len = (len + 0xFFF) & !0xFFF;
let Some(aligned_len) = len.checked_add(0xFFF).map(|v| v & !0xFFF) else {
return Err(scheme_error::EINVAL);
};

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if aligned_len == 0 {
return Err(scheme_error::EINVAL);
}
Expand All @@ -2518,7 +2523,8 @@ impl Scheme for FileSystemScheme {
crate::memory::frame_info::set_refcount(start_phys + i * 4096, 1);
}

// Zero the whole region and read file bytes into it.
// Zero the whole region (covers both file data and BSS extension), then
// copy only the file portion (`want` bytes) into the start of the buffer.
let kptr = (crate::memory::PHYS_MEM_OFFSET + start_phys) as *mut u8;
unsafe { core::ptr::write_bytes(kptr, 0, aligned_len); }
let dst = unsafe { core::slice::from_raw_parts_mut(kptr, want) };
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